use eval() to improve CPU performance (#25243)

paddle/fluid/operators/clip_by_norm_op.h

@@ -82,8 +82,13 @@ class ClipByNormKernel : public framework::OpKernel<T> {
     auto scaling = temp + (static_cast<T>(1) - temp) * max_norm / x_norm;
     Eigen::array<int, 1> one_dim{{1}};
     Eigen::DSizes<int, 1> m_dsize(input->numel());
+    if (context.GetPlace() == platform::CPUPlace()) {
+      out.device(place) =
+          x * scaling.reshape(one_dim).eval().broadcast(m_dsize);
+    } else {
       out.device(place) = x * scaling.reshape(one_dim).broadcast(m_dsize);
     }
+  }
 };
 
 class ClipByNormOp : public framework::OperatorWithKernel {